- Created by: Daanyaal
- Created on: 16-01-10 12:49
4.1 - Comparing the energy produced by fuels
Not all fuels produce same amount of energy when burnt.
To measure amount of energy produced we use instrument called a BOMB CALORIMETER.
Food = Fuel
Food can be burnt in BOMB CALORIMETER to measure amount of Energy released.
This shows how different types of food produce different amounts of energy when reacted with oxygen.
1. When fuels and food react with Oxygen, energy is released (reaction is exothermic)
2. Simple Calorimeter can be used to compare the energy released y different foods in a school chemistry lab
4.2 - Energy Changes in reactions
Exothermic Reactions - Energy released when new bonds are formed is more than energy needed to break bonds. Transfers energy TO the surroundings
Endothermic Reactions Energy needed to break bonds in reactants is more than energy released when new bonds are formed. Transfers energy FROM the surroundings TO the reacting chemicals.
Energy Level Diagrams - Show us amount of energy put in compared to amount given out.
Difference between 2 figures = ΔH (delta H). Measured in KJ/mol
Exothermic reaction - ΔH always Negative as ENERGY is RELEASED to the surroundings
Endothermic reaction ΔH always Positive as ENERGY is TAKEN IN from surroundings.
Look up Energy Lvl Diagrams further to see examples
1. In Chemical Reactions, energy must be supplied to break the bond between atoms in the reactants
2. When bonds are formed between atoms in a chemical reaction, energy is released.
3. In an EXOTHERMIC reaction the ENERGY RELEASED when bonds are formed is GREATER than the energy absorbed when BONDS are BROKEN. The opposite is true for Endothermic Reactions.
4. ΔH is negative for for exothermic reactions. It is Positive for endothermic reactions.
5. The minimum amount of energy to start a reaction is called the activation energy
5.1 Tests for Positive ions
Flame Tests (used to identify some of metals in group 1 and 2)
Place small sample of metal on platinum wire loop with concentrated HCl. Then hold loop in Bunsen burner flame and the colour will tell what Metal it is:
Lithium - Bright Red Sodium - Golden Yellow Potassium - Lilac Calcium - Brick Red
Barium - Green
Can also use unknown substances with NaOh to find what they are: Al, Ca + Mg ions all form WHITE PRECIPITATES with NaOH.
∴ if we add NaOH to unknown substance and White Precipitate forms we know its Al, Ca or Mg. If we add more and more then the Al ions dissolves but Ca and Mg wont.
Mg and Ca can be found out by flame test. Ca = Brick Red flame. Mg = no colour.
Tests for Positive ions (continued...)
NaOH Solution (continued...)
Other metals can be found by NaOH as they form colored precipitates:
NaOH + substance with COPPER (||) --> Light Blue precipitate formed
NaOH + || IRON (||) ----------> 'Dirty' Green precipitate formed
NaOH + || IRON (|||) ---------> Reddish Brown precipitate formed
Ammonia (NaOH can be used to see if Ammonia ions are present)
NaOH + Substance with ammonia ------> Ammonia + Water
Solution can be gently warmed and Ammonia driven off as a gas.
Use Red Litmus paper to detect. Red Litmus paper turns Blue as ammonia is alkaline gas.
5.2 - Tests for Negative ions
Carbonates - (dilute acid (e.g HCl) + unknown carbonate = fizzing + CO2 gas)
From here we can heat carbonate to see which metal carbonate it is:
* CuCO3 = GREEN substance. When we heat it = BLACK CuO + CO2
* ZnCO3 = WHITE substance. Heated to give ZnO = WHITE substance. ZnO heated = LEMON-YELLOW substance turning WHITE as it cools
Halides (Chloride, bromide & iodide)
dilute nitric acid + Silver Nitrate (l) to unknown solution ---> precipitate tells us halide present:
Chloride ions = WHITE precipitate Bromide ions = CREAM precipitate Iodide ions = PALE YELLOW precipitate
Tests for Negative ions (continued...)
HCl + BaCl to unknown solution ---------> WHITE PRECIPITATE shows presence of SULFATE ions. White precipitate = insoluble salt Barium Sulfate BaSO4
To detect nitrate ions. We use prev. method for Ammonia
Add NaOH to unknown + gently warm.
No ammonia detected + little aluminum powder.
Al powder reduces Nitrate ions to ammonia ions.
Ammonium ions react with NaOH forming ammonia gas. Detect using Damp Red Litmus Paper turns BLUE
5.4 - Instrumental Analysis
New machinery for detecting/ identifying elements = very popular for many industries.
e.g monitoring environment & healthcare especially (drugs in blood stream etc...)
Positives compared to older machines
* More accurate
*Small quantities can be analysed
*takes specialist training
*Results are given in comparison to other known substances
You don't need key Points!! Haven't I summarised it enough! ;)